It has been known that calcium concentration in human red blood cells control ion permeability of the membrane, as well as the shape and volume of the red blood cell. However, the details of these control mechanisms are poorly understood. Our preliminary results suggest that in human red blood cells there may be a calcium dependent regulatory mechanism similar to that in muscle protein. The goal of this project is to isolate regulatory proteins from human red cell membranes and elucidate the controlled mechanism of volume and shape of red cells.